Being Noncommittal About Sea Level Rise

A recent study by Levermann et al. (PNAS 2013) simulates the response of glaciers and ice sheets to rising global temperatures. Unlike studies that try to infer near-term sea-level rise, they look at the long-term response of the ice. They focus on two time thresholds: the equilibrium response, after everything has adjusted to the new temperatures, and the response after 2000 years, which is only partway to equilibrium.

From Levermann et al. (2013 PNAS)

I don’t have any complaints with the simulations, but the paper emphasizes an unfortunate choice of words. The title of the paper is “The Multimillennial Sea-Level Commitment of Global Warming”. The term “commitment” has been used in the context of global warming itself in the sense of how much additional warming will take place even if CO2 levels hold steady. For example, we’re “committed” to an additional 0.5-1.0 C of warming over the next several decades even if we drastically reduce emissions now.

This scenario is not one that’s likely to happen, but it’s useful because it tells us how much adaptation is necessary even under the most optimistic emission reduction scenario.

Levermann et al. use the term “commitment” to mean the same scenario: a sudden, massive drop in CO2 emissions such that CO2 levels hold steady.

The “commitment” scenario was unrealistic but useful for understanding adaptation over the next hundred years. But for the time scale that Levermann et al. talk about, it’s ridiculous.

It’s been 350 years (give or take) since the start of the industrial revolution. After another six times spans equivalent to the industrial revolution, will we not have developed technology to control atmospheric CO2 levels?

It’s been 54 years since the invention of the integrated circuit. Multiply by 37 and you have two millennia. Imagine technology advancing 37 times as much as it has over the past 54 years. Will we not have the ability to control CO2 levels by then? Some would argue that technological innovation is exponential rather than linear, in which case the relevant factor is not 37 but rather 2 raised to the 37th power, in which case our technology will have advanced by a factor of 100 billion.

We are no more committed to 2.3 m/K of sea level rise than we are committed to using internal combustion engines for the next 2,000 years.

Okay, I’ve criticized the Levermann et al. commitment scenario. What would be a realistic scenario?

I propose the Solomon et al. (2011, PNAS) scenario. Under this scenario, we continue business as usual emissions until the human race suddenly becomes extinct. (Alternatively, this could be characterized as business as usual until a comprehensive technological fix is implemented, but I prefer the pessimistic interpretation.) Rather than CO2 levels holding steady, they decline fairly rapidly at first, then slowly, with radiative forcing dropping to about 40% of its peak above preindustrial levels by sometime in the second millennium.

From Solomon et al. (PNAS 2011) showing decrease in radiative forcing and change in temperature for various greenhouse gases under the extinction scenario.

Because Levermann et al.’s sea level rise is linear with temperature and temperature is linear with radiative forcing, we can simply multiply the Levermann et al. results by 40% to get the sea level rise commitment under the Solomon et al. scenario. For example, if we put enough CO2 into the atmosphere to equilibrate global temperatures at 1 K above preindustrial levels before we become extinct, we have committed the planet to 2.3 x 0.4 = 0.9 m of sea level rise over the next two millennia.

All of this helps to put into context Climate Central’s scary interactive map of sea level rise. Look at how much of our major cities will be under water in two millennia! Ah, but under this scenario we’re extinct and the cities will be empty anyway.

If we’re not extinct, I’d hate to see what my neighborhood looks like after another 2000 years. Put another way, 2000 years is many, many infrastructure lifetimes. We will have done the equivalent of building new cities many times over in that interval. So far, our rate of abandonment of land due to toxic waste is much larger than our rate of abandonment due to sea level rise, and as long as the human race maintains its current level of intelligence, that trend will continue for a while, with no signs of major economic disruption.

My bottom line is that for all practical purposes, sea level rise 2000 years from now is simply not worth planning for.

11 Responses

I hope it’s not just me, but I find the explanation of the two papers cited pretty unclear.

Also, doesn’t what you say in the previous post (about the apparent need to track down where some 4m or so of sea level rise in the last interglacial came from) sort of undercut the reliability of the work you’re discussing here.

As for the techno-optimism: there’s no doubt that technology advances, but looking back at the predictions of writers over the last 100 years, I don’t get a lot of confidence about the accuracy of predictions about where advances will happen. Rocketry, for example, seems to me to have hit a bit of a wall, with little suggestion that alternative methods of getting heavy payloads into orbit other than by putting them on top of a can of explosives is going to arrive any time soon. (Science fiction written when I was a boy had us on the moons of Jupiter by now.) The really dramatic leap as you say is in computing. It’s amusing how rarely this was seen by science fiction: you have Heinlein in the 50′s with astronauts using slide rules, and even by 1974, in the popular Niven/Pournelle book The Mote in God’s Eye, astronauts in an advanced star faring civilisation a thousand years off are using handheld computers which are now commonplace in the form of smartphones and tablets.

But how relevant is this technology for decreasing CO2 already released? You’re basically talking about large scale chemical or biological means of re-burying it at a rate massively above how nature would deal with it. I’m not sure that improved technology can get over the fundamentals of how long it takes to put an entire atmosphere through a cleaning process.

So I feel rather skeptical about the “we’ll be able to deal with this in 500 years” approach.

Overall, the paper isn’t very good on dealing with the uncertainties, but I’m taking the paper at face value and looking at its implications.

My view is that rocketry has hit a wall because the economic incentive is small. Science fiction writers imagine the possible, not the practical. We’re also not limited to CO2 removal as a solution, though admittedly the economic incentive for geoengineering hasn’t materialized yet either.

Steve, you suggested: “large scale chemical or biological means of re-burying it [CO2] at a rate massively above how nature would deal with it. I’m not sure that improved technology can get over the fundamentals of how long it takes to put an entire atmosphere through a cleaning process.”

Actually, I see several reasons to be technically optimistic. First, catalytic chemistry is improving quickly, and increasingly efficient means of CO2 trapping are thereby not unreasonable to expect. Second, one doe not need to remove CO2 “massively above” how nature would deal with it; just tilt the balance toward removal. Third, both biological and chemical means of CO2 trapping and burying can be jointly used; and fourth, as John N-G points out, the “500-year” time window might easily be lengthened by interventions other than CO2 removal, e.g., lofting reflectors into space to reduce solar warming.

Of course, unpredcted events could still alter the projections. For example, solar output of energy might dip, unexpectedly. Human population might fall, for whatever reason. Or we might decide that underwater habitats are both technically feasible and desirable.

I can agree with your bottom line Dr. n-g. And would add, as a paleoclimatologist, when I see and hear people worrying about .9 meters rise over 2000 years, or even 2 or 3 meters over a quarter of that time, I have to laugh at their ignorance, they obviously have no education into what has happened due to Mother Nature’s forces (much stronger than human CO2) recently:

Some have argued we’re well into overshoot being a detritovore, and liable to crashes from relatively minor interruptions — because most every human life these days depends on the continuing operation of things that can’t continue forever, but were cheap to build because there was this continent to consume.
The Americas are about consumed, barring the Amazon.

The input from colonizing this large an area that was very lightly used — is over, pretty much, and most of the basic inputs — phosphorous to the land, iron to the sea — were animal mediated. Seabirds, guano, mostly gone. Whales, iron, mostly gone.

Innovation’s very nice, but innovation won’t get you through periods of low primary productivity well at all. And we’re knocking the pins out of that.

Ask an ecologist about Aldo Leopold’s “Land Ethic” — which has to be extended to the air and the oceans, we know know. We’ve just about finished eating and burning what we should’ve been building up.

Under this scenario, we continue business as usual emissions until the human race suddenly becomes extinct.

Not very likely. What’s much, much more likely is that the atmosphere’s CO2 concentration will be returned to whatever is decided to be the “optimum” level (perhaps 350 ppm?) sometime in the latter half of the 21st century or first half of the 22nd century.

10% of GDP? Seriously? There are quite a few Western countries that spend less on healthcare than that. To use such enormous amounts of money on one thing and one thing only, and by all countries together, will require a global dictatorship.

Note also that the melting of the ice caps won’t just magically stop right away when temperatures start to drop from the higher level at the end of the 21st century. This will likely continue for a few hundred if not thousand years.

So, costs of CO2 removal will not only have to drop dramatically, the active CO2 removal will have to be implemented as soon as possible.